1. Field of the Invention
The present invention relates to a high-frequency discharge ignition apparatus that flows high-frequency AC current into a spark discharge path to cause discharge plasma in a main plug gap, thereby igniting an internal combustion engine.
2. Description of the Background Art
In recent years, problems of environmental conservation and fuel depletion have been raised, and there is an urgent need to address such problems also in automobile industry.
As an example of methods for addressing such problems, there is a method of dramatically improving fuel consumption by engine downsizing and weight reduction by using a supercharger.
It is known that in a highly supercharged state, the pressure in an engine combustion chamber becomes very high even without combustion, so that it becomes difficult to cause spark discharge for starting combustion in the chamber in this state.
One of the reasons is that required voltage for causing insulation breakdown (in gap) between a high voltage electrode and a GND (ground) electrode of an ignition plug increases so much that the required voltage exceeds the withstand voltage value of an insulator portion of the ignition plug.
In order to solve this problem, studies for increasing the withstand voltage of the insulator portion are conducted, but actually, it is difficult to ensure a sufficient withstand voltage for the requirement, and therefore there is no choice but to employ means of narrowing the gap interval of the ignition plug.
However, if the gap of the ignition plug is narrowed, an effect of quenching action by an electrode portion increases, and this causes a problem of reducing startability and reducing combustibility.
In order to solve this problem, it is conceivable as avoidance means to provide spark discharge with energy exceeding heat to be taken by the electrode portion by quenching action or to cause combustion at a position as far from the electrode as possible. For example, an ignition coil apparatus as shown in Japanese Laid-Open Patent Publication No. 2012-112310 (hereinafter, referred to as Patent Document 1) is proposed.
The ignition coil apparatus disclosed in Patent Document 1 causes spark discharge in a gap of an ignition plug by a conventional ignition coil and flows high-frequency current into a path for spark discharge via a mixer using a capacitor, thereby making it possible to cause spark discharge with high energy and discharge plasma spreading more widely than in normal spark discharge.
The conventional ignition coil apparatus shown in Patent Document 1 separates or couples a high voltage system and a large current system by using a high withstand voltage capacitor.
Generally, a capacitor has a temperature characteristic, and its permissible value varies in accordance with variation in the environmental temperature.
Since the conventional ignition coil apparatus shown in Patent Document 1 applies AC current according to the pass frequency band of the capacitor to the path for spark discharge, the level of current to be applied to the path for spark discharge greatly varies by variation in the characteristic of the capacitor due to the temperature.
Therefore, it is conceivable to detect the current level and thereby control the frequency of the AC current according to the pass frequency band of the capacitor.
However, if the combustion state of the internal combustion engine varies so that the combustion state becomes unstable, the impedance of the spark discharge path increases, and therefore the level of current to be applied to the path for spark discharge also varies greatly, thus causing a problem that the current cannot be applied stably.